(1) Field of the Invention
The present invention relates to a fixed-length cell transmission system, a fixed-length cell transmitting unit, and a fixed-length cell receiving unit, and more particularly to a fixed-length cell transmission system, a fixed-length cell transmitting unit, and a fixed-length cell receiving unit suitable for an ATM (Asynchronous Transfer Mode) optical transmission technique (an asynchronous optical transmission network) which deals with fixed-length data (cell) called an ATM cell.
As well known, the ATM transmission technique has become a center of attraction as a means capable of realizing a broad-band communication network, and particularly with the recent progress of the optical transmission technique, interest has been shown toward, for example, the ATM optical transmission technique based upon an optical wavelength multiplexing technique which allows an effective utilization (band broadening) of an operating (using) frequency band per one optical fiber by wavelength-multiplexing data (ATM cell) including a plurality of different light-wavelength information.
(2) Description of the Related Art
FIG. 15 is a block diagram showing one example of ATM cell transmission system, and as illustrated, the ATM cell transmission system, for example, has a demultiplexing unit 101 located on a center side and equipped with an ATM exchange (not shown) and a multiplexing unit 102, called an RT (Remote Terminal), situated on a subscriber (user) side.
Furthermore, as shown in FIG. 15, the demultiplexing unit (fixed-length cell transmitting unit) 101 is composed of a demultiplexing section (DMUX) 103 and a plurality of electric/optic conversion sections (E/O) 104-1 to 104-n (where n designates a natural number), whereas the multiplexing unit (fixed-length cell receiving unit) 102 includes optic/electric conversion sections (O/E) 105-1 to 105-n respectively corresponding to the electric/optic conversion sections (E/O) 104-1 to 104-n and a multiplexing section (MUX) 106.
In this system, the demultiplexing section 103 of the demultiplexing unit 101 is for the purpose of separating or demultiplexing input data to distribute ATM cell data received in the form of a metallic signal (electric signal) from the ATM exchange side to the electric/optic conversion sections 104-i (where i=1 to n) in accordance with its VPI (Virtual Path Identifier)/VCI (Virtual Channel Identifier). Each of the electric/optic conversion sections (optical transmitting sections) 104-i converts input data into an optical signal having a given wavelength .lambda.i.
In this case, the data converted into the optical signals in the electric/optic conversion sections 104-i are wavelength-multiplexed and transmitted through one optical transmission line 107.
On the other hand, in the multiplexing unit 102, each of the optic/electric conversion sections (optical receiving sections) 105-i converts the optical signal with the wavelength .lambda.i, inputted through the optical transmission line 107, into a metallic signal, while the multiplexing section 106 outputs the metallic signals (ATM cell) from the respective optic/electric conversion sections 105-i in a time-division way for multiplexing, with the multiplexed signals being forwarded to the user side.
A brief description of the aforesaid ATM cell transmission system (ATM transmission technique based upon the light-wavelength multiplexing technique) is to say that, for the optic-electric conversion of the data (ATM cell) to be transmitted, the entire operating band is divided into the transmission rates (bands .lambda.1 to .lambda.n) of the optical transmitting sections 104-i (the optical receiving sections 105-i) and transmitted. In this case, the whole transmission rate becomes constant irrespective of the presence or absence of information.
With such an arrangement, the above-described ATM cell transmission system can accomplish the transmission of data from the center side through the optical transmission line 107 between the demultiplexing unit 101 and the multiplexing unit 102 with an extremely high efficiency. Accordingly, for example, if the multiplexing unit 102 is disposed in the vicinity of the user, the data from the center can advance through the optical transmission line 107 to the vicinity of the user with almost no loss.
On the other hand, since in general the ATM transmission technique makes the operating band vary on all such occasions, the optical section equivalent to the physical layer portion may allow data transmission without the need for use of all the optical transmitting sections 104-i under certain circumstances. However, since the rule of the above-mentioned ATM cell transmission system is that the supply of information falling under a constant band always comes about, it needs such an operation as to send a vacant cell from a portion of the optical transmitting sections 104-i. This not only leads to increase in power consumption of the whole system but also makes it difficult to effectively make the most of the flexible transmission technique relying upon the VP/VC producing the characteristic of the ATM.